A gigantic storm is set to hit the West Coast later today, bringing with it over a foot of rain. It's what meteorologists call an Atmospheric River—but what is that, exactly?

An atmospheric river is a meteorological phenomenon…

You guessed it! It's not a river at all. It's a weather event, a serious one, that happens all the time over oceans—but can be catastrophic when hitting land.

...when weather fronts blow in different directions...

Weather systems don't neatly blend into one another. You probably know that when high and low pressure weather systems meet they can combine to form a storm front—but that's not the whole story. When two weather systems are dominated by winds that are moving away from each other, what meteorologists would call divergent surface air flow, they leave an empty space between them, which is just waiting to be filled.

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...moisture can become concentrated in long, thin corridors...

When a gap between those two diverging weather fronts appears, it provides a speedy route for water to ascend into the atmosphere. Think of it as a low pressure void that can't help but suck up whatever's beneath it—and if it happens to over the sea, it will naturally suck up huge quantities of water vapor.

...which carry unbelievable amounts of water...

And when we say huge, we mean huge. Atmospheric rivers can grow and grow and grow... until they're hundreds of kilometers wide and thousands of kilometers long. In fact, they can often carry more water than the Amazon. Yes, the world's biggest river. Which is why, despite the fact that you may never have heard of an atmospheric river, they actually account for 90 percent of the world's north-south water vapor transport.

... and can shed their payload in dramatic style.

Which means that, occasionally, they wreak havoc. Usually these things are confined to the air above oceans; there are often up to ten floating around the planet at any given moment. But if they wind up hitting shore, they can bring a hell of a lot of rain with them: in fact, they're the main culprit behind severe flooding caused by rainfall, at least in mid-latitude areas of the western world. In San Francisco's case, that could mean up to a foot of rain over the next few days. Brace yourself, California.